1. Field of the Invention
The present invention relates to a method and a device for measuring gas or liquid concentration in a binary mixture formality according to the preamble of the independent claims. The present invention also provides the methods and process for fabricating the device with micromachining or Micro Electro Mechanical Systems (MEMS) approach.
2. Description of the Related Art
Various gas or liquid concentration meters have been heretofore developed and commercially available on the market. The gas concentration sensors are broadly deployed in the fields such as oxygen, nitrogen, and other inert gas concentration detection. The liquid concentration sensors are applied on beverage, pharmaceutical, and chemical industries etc. The operation principles behind these commercial products are mainly based on the methods such as electrochemical reaction (H. Weyl and B. Wild, Measuring Device, U.S. Pat. No. 6,039,856; H. Dietz, Polarographic Oxygen Concentration Sensor and Method of Determining Oxygen Content in the Exhaust Gases of an Internal Combustion Engine, U.S. Pat. No. 4,356,065; G. Richter, G. Luft, U. Gebhardt, Method for Determining the Concentration of Sugar Using an Electrocatalytic Sugar Sensor, U.S. Pat. No. 4,366,033); optical refraction index (S. Akiyama, M. Fujiwara, T. Oida, et al., Gas Analyzer, U.S. Pat. No. 5,773,828; A. Robinovich, E. Diatzikis, J. Mullen, D. Tulimieri, Infrared Sensing of Concentration of Methanol's Aqueous Solution, U.S. Pat. No. 6,815,682); ultrasonic acoustic wave (A. Rabinovich and D. Tulimieri, Ultrasound Sensing of Concentration of Methanol's Aqueous Solution, U.S. Pat. No. 6,748,793); vibration resonate frequency (G. A. Michaels and H. Birangi, Gas Concentration Sensor and Control for Oxygen Concentrator Utilizing Gas Concentration Sensor, U.S. Pat. No. 5,917,135); capacitance detection (Richard K. Rader et al, Alcohol Concentration Sensor for Automotive Fuels, U.S. Pat. No. 5,255,656) and Coriolis force (F. C. Sittler, J. H. Crabtree, Fluid Flow Detector, U.S. Pat. No. 4,909,078) measurement.
U.S. Pat. No. 4,902,138 (Heinz-Dieter Goeldner, Measurement Component Concentration in a Gas Blend) reveals a device that a gas blend is introduced to a micro-machined chamber to indirectly determine its component concentration through measuring the thermal conductivity of gas blend (see FIG. 1(a)). The thermal conductivity sensor is composed of two finger-interlaced serpentine resistors wherein one of the resistors is heated up by a control circuit to elevate the temperature of proximate gas blend, and the other resistor is utilized to sense the temperature variation of gas blend. By measuring the gas blend temperature at different power level of heating resistor, the respectively collected data could apply to solve one set of equations, so the individual concentration of gas components can be determined. One of the drawbacks in this invention is the direct thermal conduction between the heating and sensing resistors could affect the accuracy of measurement since they are disposed so closely. In an ideal situation, the sensing resistor should only receive the heat conduction from gas blend. On the other hand, the sensitivity may become inferior if the heating and sensing resistors are separated further. Another drawback is that the invention can only limit to function in a static gas blend situation. The device in this invention could not work in a dynamic flow situation completely.
Zemel et al. teaches the approaches to measure mass flow and thermal conductivity simultaneously (Simultaneous Measurement of Gas Thermal Conductivity and Mass Flow, U.S. Pat. No. 5,463,899). Referring to FIG. 1(b), the system comprises two pyroelectric anemometers which are disposed respectively within each of the two conduits in a concentric arrangement. Each pyroelectric anemometer is connected to a differential amplifier. Since the ratio of the output signal of each pyroelectric anemometer in conduit 1 and conduit 2, I1 and I2 can be represented as
                    I        2        2                    I        1              =                  x        gas            ×      const        ,where xgas is gas thermal conductivity and const is a function of geometry which could be measured and decided. Therefore, the thermal conductivity of gas could be derived once I1 and I2 are obtained. The gas velocity can be correspondingly derived by formula from the known thermal conductivity xgas. One of the downsides of this invention is the anemometer has to limit to pyroelectric type or the equation to derive the gas thermal conductivity is no longer true. Not like the conventional hot-wire anemometer, the pyroelectric anemometer needs high pyroelectric material (e.g. LiTaO3) as substrate. The other drawbacks of this invention are coming from the requirements of double sensors, which are more costly, and the complexity of sensors disposition.
Therewith, the current invention shall have properties in many aspects of differentiation include dynamic metrology capability, faster response, lengthy lifetime, easiness of integration and lower cost.